In this one, Ant-Man isn't around yet so it looks like Tony Stark creates the Ultron A.I. to help out the Avengers and uses old Iron Man armor as a body. Ultron then decides HE'S in charge and everything goes to hell.

Ultron is made of/coats himself in vibranium, which is what Captain America's shield is made out of. Thus rendering himself nearly invincible. Big fights, blah blah blah. EXPLOOOOOOOOOSIONS!